Georgia Institute of Technology, United States

The widespread presence of bottom-simulating reflectors (BSRs) on continental margins has bolstered suggestions that gas hydrates and free gas constitute a large dynamic reservoir of CH (sub 4) carbon and a vast potential source of energy. However, only a few hydrate-bearing areas have been drilled, and of these, the amount of CH (sub 4) has only been directly quantified in 18 discrete samples from 3 holes on Blake Ridge, east of Georgia. Here we report and discuss 30 direct measurements of CH (sub 4) concentration in sediments above and below the BSR at Hydrate Ridge on a tectonically active margin offshore Oregon. High CH (sub 4) concentrations (71-3127 mM) support abundant gas hydrate (occupying an average of approximately 11% of porosity) and free gas (occupying approximately 4% of porosity in 1 sample) in a restricted area where hydrocarbon gases migrate from the deep accretionary complex to the seafloor. In a larger area lacking this hydrocarbon supply, lower CH (sub 4) concentrations (10-893 mM) indicate less gas hydrate (average approximately 1% of porosity) and little or no free gas. Overall, the amount of CH (sub 4) at Hydrate Ridge is significantly less than that at Blake Ridge. These results challenge certain interpretations, including the global volume of hydrate-bound CH (sub 4) , which though large, may be four to seven times less than widely cited estimates. Speculations on the distribution and role of gas hydrate and free gas need revision.